Magneto-optical microscopy and magnetometry have been used to study\ud 19 magnetization reversal in an ultrathin magnetically soft [Pt/Co]2 ferromagnetic film\ud 20 coupled to an array of magnetically harder [Co/Pt]4 nanodots via a predominantly\ud 21 dipolar interaction across a 3 nm Pt spacer. This interaction generates a spatially\ud 22 periodic pinning potential for domain walls propagating through the continuous\ud 23 magnetic film. When reversing the applied field with respect to the static nanodot\ud 24 array magnetization orientation, strong asymmetries in the wall velocity and switching\ud 25 fields are observed. Asymmetric switching fields mean that the hysteresis of the film is\ud 26 characterized by a large bias field of dipolar origin which is linked to the wall velocity\ud 27 asymmetry. This latter asymmetry, though large at low fields, vanishes at high fields\ud 28 where the domains become round and compact. A field-polarity-controlled transition\ud 29 from dendritic to compact faceted domain structures is also seen at low field and a\ud 30 model is proposed to interpret the transition.